OBJECTIVE： To prepare and characterize Fluorescent dye 1，1′ -octacosyl-3，3，3′ ，3′ -tetramethylindocarbocyanine iodide（DiR）-loading polyethylene glycol-poly lactic-co-glycolic acid （DiR-PEG-PLGA） nanocapsules， and to evaluate its biocompatibility in vitro. METHODS： Using PLGA and PEG-PLGA as carrier， DiR-PEG-PLGA nanocapsules were prepared by modified ultrasonic emulsification method. The particle size， Zeta potential， morphology， stability and fluorescence in vitro of nanocapsules were detected respectively. MTT assay was used to evaluate cytotoxicity in vitro of nanocapsules to human-derived  HL7702 hepatocytes， and hemolysis test was carried out to investigate its hemolysis effects. RESULTS： Prepared DiR-PEG-PLGA nanocapsules were spherical with a clear core-shell structure. The average particle size was （507.53±7.87） nm， polydispersity coetficient of particle size was 0.306 1±0.001 5 and Zeta potential was （－35.20±0.92） mV with good stability within 6 months under 4 ℃. Fluorescence signal intensity （y） of nanocapsules was increased linearly with DiR mass concentration （x） in vitro. The linear eguation was y＝0.345 2x+0.433 4（R2＝0.997 3）. The toxicity of nanocapsules to HL7702 cells was between 0-1 degree，and no hemolytic effect was observed. CONCLUSIONS： The study successfully prepare fluorescent DiR-PEG-PLGA nanocapsules with high biocompatibility in vitro， which is further expected to become a safe optical drug carrier.